The present invention relates to a ratio change control for a transmission, and more particularly to a ratio change control for a hybrid continuously variable transmission including a gearing mechanism and a continuously variable transmission mechanism which are selectively rendered operabble to take over a drive from a transmission input shaft to a transmission output shaft.
U.S. Pat. No. 4,735,113 discloses a V-belt type continuously variable transmission including a driver pulley, a follower pulley, and a V-belt drivingly interconnecting these pulleys. Laid-open Japanese Patent Application No. 63-176862 and U.S. Pat. No. 4,907,471 disclose a hybrid continuously variable transmission in which the above-mentioned continuously variable transmission mechanism is combined with a gearing mechanism such that the gearing mechanism is put into operation to provide a reduction ratio between a transmission input shaft and a transmission output shaft for start-up operation. The setting is such that this reduction ratio provided by the gearing mechanism is larger than the maximum or largest reduction ratio provided by the continuously variable transmission mechanism. For high speed operation where a relatively small drive force is required, the continuously variable transmission mechanism is put into operation to take over the drive from the transmission input shaft to the transmission output shaft owing to engagement of a power interruption device, such as a clutch.
Referring to FIG. 9, consider the case where the control strategy shown in U.S. Pat. No. 4,735,113 is applied to the hybrid continuously variable transmission of the type mentioned above. FIG. 9 is a graph wherein a line G shows target driver pulley revolution speed values versus varying vehicle speed values when a gearing mechanism is put into operation to take over a drive. In FIG. 9, line L shows target driver pulley revolution speed values versus vehicle speed values when a continuously variable transmission mechanism provides its maximum or largest reduction ratio, while line S shows target driver pulley revolution speed values versus vehicle speed values when the continuously variable transmission mechanism provides its minimum or smallest reduction ratio. Let it be assumed that a vehicle starts moving with a 3/8 throttle opening degree. In this case, the driver pulley revolution speed increases along the line G from a point O to a point A with the reduction ratio provided by the gearing mechanism. Upon arriving at point A, a transition is made from the drive owing to the gearing mechanism to a drive owing to the continuously variable transmission. This causes a drop in driver pulley revolution speed without any substantial change in vehicle speed, resulting in a transfer from the point A to a point B on the line L. Then, with the maximum reduction ratio provided by the continuously variable transmission mechanism, the vehicle increases its speed till a point C on the line L. As the vehicle increases its speed further from a vehicle speed value corresponding to the point C, the reduction ratio decreases continuously toward the minimum reduction ratio provided by the continuously variable transmission mechanism along an operation line I. According to this control strategy, the continuously variable transmission mechanism effects a ratio change from the maximum reduction ratio to a relatively smaller reduction ratio on a broken line M, since a target driver pulley revolution speed corresponding to the point C' is set if the vehicle speed increases to a vehicle speed value corresponding to the point C'. A problem arises if, during transition from the drive owing to the gearing mechanism to the drive owing to the continuously variable transmission mechanism, the vehicle speed increases beyond the vehicle speed value corresponding to the point C and the continuously variable transmission has shifted off the maximum reduction ratio thereof. The transition in drive takes a relatively long time, since there is a delay in the hydraulic system for activating a clutch contributing to this transition. In this case, the vehicle starts running through the drive owing to the continuously variable transmission mechanism with a reduction ratio smaller than the maximum reduction ratio thereof. This transition inherently causes a substantial shock, which causes an insufficient driving force and thus poor acceleration.
Another hybrid continuously variable transmission is known. This known hybrid continuously variable transmission has a gearing mechanism which provides a reduction ratio smaller than the minimum reduction ratio provided by a continuously variable transmission mechanism. Such a hybrid continuously variable transmission is disclosed in Laid-open Japanese Patent Application No. 58-156764. If the continuously variable transmission mechanism is controlled according to the above-mentioned control strategy, there occurs a case where the continuously variable transmission mechanism shifts down from the minimum reduction ratio thereof during a transition from a drive owing to the gearing mechanism to a drive owing to the continuously variable transmission mechanism. This causes a substantial increase in engine speed, inducing a substantial shock.
An object of the present invention is to provide a ratio change control for a transmission which assures smooth and shockless transition between a drive owing to a gearing mechanism and a drive owing to a continuously variable transmission mechanism.
More specifically, the present invention provides a ratio change control employing a control action including a feedback control term in addition to a feedforward control term.